Fourier transform infrared (FTIR) interferometric spectrometers are widely used in the analysis of chemical compounds. By measuring the absorption of infrared radiation by an unknown sample at various wavelengths in the infrared spectrum and comparing the results with known standards, these instruments generate useful information with respect to the chemical makeup of the unknown sample. In a typical FTIR spectrometer, infrared radiation from an infrared emitting source is collected, passed through an interferometer, passed through the sample to be analyzed, and brought to focus on an infrared detector. The interferometer system, in combination with the sample, modulates the intensity of the infrared radiation that impinges on the detector, and thereby forms a time variant intensity signal. It is the function of the detector to convert this time variant intensity signal to a corresponding time varying current. The current, in turn, is converted to a time varying voltage, which is presented to an analog-to-digital converter and then stored as a sequence of digital numbers to be processed in a processor associated with the spectrometer.
One important feature of the FTIR spectrometer is the moving mirror element that modulates the analytical radiation used by the instrument to study samples. The moving mirror allows a time-domain interferogram to be generated which, when analyzed, allows high resolution frequency-domain spectra to be produced. The computer performs a Fourier transform on the data to produce a spectrum which shows spectral-energy versus frequency.
It is critical in the design of these instruments that the surface of the moving mirror be very accurately held in an orthogonal position, i.e., at a right angle, to the direction of the motion of the moving mirror. Positional accuracy of the moving mirror is crucial because deviations in the mirror alignment produce small errors in the time-domain interferogram that may translate into large errors in the frequency-domain spectrum. In a typical interferometer, deviations of the moving mirror larger than one wavelength of the analytical radiation used are considered significant and can seriously degrade the quality of the entire instrument.
In one type of interferometer, the moving mirror is mounted upon an arm that is guided along its line of motion by suspension from two links spaced a distance apart. The links are pivotably attached to the mirror arm and to the housing of the interferometer at pivots, thus forming a parallelogram. Such an arrangement is sometimes referred to as a "porch swing" style of interferometer because of the likeness of its movement to its namesake.
Where the parallelogram formed by the mirror arm, links, and housing is not true, i.e. the links are not of equal length or not in the same plane of motion, the moving mirror may be tilted along its path of motion, thus introducing errors into the spectrum. The two horizontal members (the housing and the mirror arm) must be of equal length, as must the vertical members (the links), in order to keep the mirror from tilting during travel. One prior art design uses adjusting collets at one of the pivot points along the mirror arm to fine tune the lengths of the horizontal and vertical members. There are, however, problems associated with the use of such collets to make adjustments to correct for mirror tilt. For example, the adjustments are made on the moving portion of the mechanism, so that the mirror motion must be stopped for each adjustment iteration and then restarted to check the resulting tilt. The collet adjustment is non-linear, so that the results of each adjustment iteration are not necessarily predictable. Finally, the collets are held in place by set screws that must be loosened and re-tightened with each adjustment. However, the tightening of the set screws themselves affects the alignment.
The prior art means of adjusting the tilt in a moving mirror of an interferometer is therefore inaccurate, slow, and awkward. Accordingly, a need has arisen for a mechanism to adjust the tilt of a moving mirror that is accurate and efficient.